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WASHINGTON (Reuters) - U.S. and British military officials are working on a joint directive to require mandatory inspections of engines on all Lockheed Martin Corp F-35 fighter jets, after an Air Force F-35A caught fire at a Florida air base last week, said sources familiar with the situation.

That incident was the latest to hit the Pentagon's costliest weapons program, the $398.6 billion (£232.2 billion) F-35 Joint Strike Fighter, and came as the plane was preparing for its international debut at two air shows in Britain.

The incident involved the third stage of the F135 engine built by Pratt & Whitney, a unit of United Technologies Corp, for all three models of the new warplane, the sources said. "The engine ripped through the top of the plane," one said.

Details are still being finalised, but the inspections could take about 90 minutes, according to one of the sources, who was not authorized to speak publicly.

The incident will prevent a planned F-35 "fly by" at the July 4 naming ceremony of Britain's new aircraft carrier.

It has stalled the departure of the planes that were to participate in the UK shows, but the U.S. Marine Corps said it was still planning to send four jets across the Atlantic.

"We are on track to participate in the air shows," said Marine Corps Captain Richard Ulsh.

The Royal International Air Tattoo (RIAT) kicks off on July 11 followed by the Farnborough International Air Show starting on July 14.

Four Marine Corps F-35 B-model jets remain in southern Maryland waiting to leave for Britain. A fifth jet, owned by Britain, is still in Florida.

Strict UK liability laws mean British authorities need additional information before granting flight clearances. If something went wrong, the individual officer who approved the flight could be sued personally, the sources said.

Pratt & Whitney declined comment on any finding of the ongoing investigation into the fire at the Florida air base. Company spokesman Matthew Bates said the company was participating in the investigation.

One of the sources said the engine involved - and about six feet of debris found on the runway around the jet - were shipped to Pratt's West Palm Beach facility on Tuesday for a more detailed inspection.

Early indications pointed to a possible quality problem with one part of the engine, but that finding must still be further corroborated, said one of the sources.